The invention relates to fault-tolerant storage of computer data.
Known computer backup methods copy files from a computer disk to tape. In a full backup, all files of the disk are copied to tape, often requiring that all users be locked out until the process completes. In an "incremental backup," only those disk files that have changed since the previous backup, are copied to tape. If a file is corrupted, or the disk or its host computer fails, the last version of the file that was backed-up to tape can be restored by mounting the backup tape and copying the backup tape's copy over the corrupted disk copy or to a good disk.
Data can also be protected against failure of its storage device by "disk mirroring," in which data are stored redundantly on two or more disks.
In both backup systems and disk mirroring systems, a program using a restored backup copy or mirror copy may have to be altered to refer to the restored copy at its new location.
In hierarchical storage systems, intensively-used and frequently-accessed data are stored in fast but expensive memory, and less-frequently-accessed data are stored in less-expensive but slower memory. A typical hierarchical storage system might have several levels of progressively-slower and -cheaper memories, including processor registers, cache memory, main storage (RAM), disk, and off-line tape storage.